The invention relates to a capacitively coupled radiofrequency (RF) plasma reactor and to a process for treating at least one substrate in such a reactor. Especially, the present invention applies to a large size capacitive capacitively coupled (RF) plasma reactor.
Often, such a reactor is known as a "capacitive" RF glow discharge reactor, or planar plasma capacitor or parallel plate RF plasma reactor, or as a combination of the above named. Capacitive RF plasma reactors are typically used for exposing a substrate to the processing action of a glow discharge. Various processes are used to modify the nature of the substrate surface. Depending on the process and in particular the nature of the gas injected in the glow discharge, the substrate properties can be modified (adhesion, wetting), a thin film added (chemical vapour deposition CVD, diode sputtering) or another thin film selectively removed (dry etching). The table shown below gives a simplified summary of the various processes possibly performed in a low pressure capacitive discharge.
Industry Substrate type Process Inlet gas nature Semiconductor wafer - Surface Cleaning - Ar up to 30 cm - PECVD - SiH.sub.4, . . . diameter - Dry Etching - CF.sub.4, SF.sub.6, Cl.sub.2, . . . - Ashing - O.sub.2, Disks for Polymeror glass - Diode sputtering - Ar + others memory up to 30 cm - PECVD - Organometallics diameter - Surface activation - O.sub.2, etc. . . . Flat display Glass Same as for Same as for up to 1.4 m diagonal semiconductors semiconductors Window pane Glass up to 3 m - Cleaning/ - Air, Argon Web coaters width, foil, plastic activation, Monomer, Nitrogen, or metal Nitriding, polymer . . . PECVD
The standard frequency of the radiofrequency generators mostly used in the industry is 13.56 MHz. Such a frequency is allowed for industrial use by international telecommunication regulations. However, lower and higher frequencies were discussed from the pioneering days of plasma capacitor applications. Nowadays, for example for PECVD applications, (plasma enhanced chemical vapour deposition) there is a trend to shift the RF frequency to values higher than 13.56 MHz, the favourite values being 27.12 MHz and 40.68 MHz (harmonics of 13.56 MHz). So, this invention applies to RF frequencies (1 to 100 MHz range), but it is mostly relevant to the case of higher frequencies (above 10 MHz). The invention can even be applied up to the microwave range (several GHz).
An important problem was noted especially if the RF frequency is higher than 13.56 MHz and a large size (surface) substrate is used, in such a way that the reactor size is no more negligible relative to the free space wave length of the RF electromagnetic wave. Then, the plasma intensity along the reactor can no longer be uniform. Physically, the origin of such a limitation should lie in the fact that the RF wave is distributed according to the beginning of a "standing wave" spacial oscillation within the reactor. Other non uniformities can also occur in a reactor, for example non uniformities induced by the reactive gas provided for the plasma process.
It is an object of the invention to propose a solution for eliminating, or at least notably reducing, an electromagnetic (or a process) non uniformity, in a reactor. Thus, according to an important feature of the invention, an improved capacitively coupled RF plasma reactor should comprise:
at least two electrically conductive electrodes spaced from each other, each electrode having an external surface, PA1 an internal process space enclosed between the electrodes, PA1 gas providing means for providing the internal process space with a reactive gas, PA1 at least one radiofrequency generator connected to at least one of the electrodes, at a connection location, for generating a plasma discharge in the process space, and potentially an additional RF generator for increasing the ion bombardment on the substrate, PA1 means for evacuating the reactive gas from the reactor, so that said gas circulates within the reactor, at least in the process space thereof, PA1 at least one substrate defining one limit of the internal process space, to be exposed to the processing action of the plasma discharge, said at least one substrate extending along a general surface and being arranged between the electrodes, PA1 locating the at least one substrate between at least two electrodes, the substrate extending along a general surface, PA1 having a reactive gas (or gas mixture) in an internal process space arranged between the electrodes, PA1 having a radiofrequency generator connected to at least one of the electrodes, at a connection location, PA1 having a plasma discharge in at least a zone of the internal process space facing the substrate, in such a way that said substrate is exposed to the processing action of the plasma discharge, PA1 creating an extra-capacitor electrically in series with the substrate and the plasma, said extra-capacitor having a profile, and PA1 defining the profile of the extra-capacitor in such a way that it has location dependent capacitance per unit surface values along at least one direction of the general surface of the substrate. PA1 the so-called "corrective layer" is the thickest in front of the location in the process space (where the plasma is generated) which is the farest away from the connection location where the radiofrequency generator is connected to said at least one electrode, the distance being measured by following the electrode external surface, PA1 and said thickness preferably decreases from said process space location, as the distance between the process space location and the connection location on the corresponding electrode decreases. PA1 a capacitor is introduced between the electrodes, said capacitor being in series with the plasma (and the substrate) in the reactor, PA1 this extra-capacitor acts with the plasma capacitor itself as a voltage divider tailoring the local RF power distribution, to (substantially) compensate a non uniformity of the process due, for example, to gas compositional non uniformity, to edge effects or to temperature gradient.
characterized in that it further comprises at least one dielectric "corrective" layer extending outside the internal process space, as a capacitor electrically in series with said at least one substrate and the plasma, said at least one dielectric layer having capacitance per unit surface values which is not uniform along at least one direction of said general surface, for compensating a process non uniformity in the reactor or to generate a given distribution profile.
In other words, the proposed treating process in the reactor of the invention comprises the steps of:
It is to be noted that such a solution is general. It is valid for all plasma processes, but only for a determined RF frequency.
The "tailored extra-capacitor" corresponding to the above-mentioned said (substantially) "dielectric layer" acts as a component of a capacitive divider. Advantageously, the capacitive variations will be obtained through a non uniform thickness of the layer. Thus, the extra-capacitor will have a profile having a non planar-shape along a surface.
For compensating a non uniform voltage distribution across the process space of the reactor, said thickness will preferably be defined in such a way that:
Of course, it is to be understood that the above-mentioned "distance" is the shortest of all possible ways.
So, if the electromagnetic travelling waves induced in the process space combine each other near the center of the reactor to form a standing wave having a maximum of voltage in the vicinity of the reactor center, the thickness of the so-called "corrective layer" will be larger in the vicinity of the center thereof, than at its periphery.
One solution in the invention for tailoring said "corrective layer" is to shape at least one surface of the layer in such a way that the layer has a non planar-shaped external surface, preferably a curved concave surface facing the internal process space where the plasma is generated. Various ways can be followed for obtaining such a "non planar shaped" surface on the layer.
It is a priviledged way in the invention to shape at least one of the electrodes, in such a way that said electrode has a non planar-shaped surface facing the substrate, and especially a generally curved concave surface. It is another object of the invention to define the composition or constitution of the so-called "corrective layer".
According to a preferred solution, said layer comprises at least one of a solid dielectric layer and gaseous dielectric layer. If the layer comprises such a gaseous dielectric layer, it will preferably be in gaseous communication with the internal process space where the plasma is generated. A substrate comprising a plate having a non planar-shaped external surface is also a solution for providing the reactor of the invention with the so-called "corrective layer".
Another object of the invention is to define the arrangement of the substrate within the reactor. Therefore, the substrate could comprise (or consist in) a solid member arranged against spacing members located between said solid member and one of the electrodes, the spacing member extending in said "corrective layer" along a main direction and having, each, an elongation along said main direction, the elongations being non uniform along the solid member. A difficulty induced by such spacing members relates to a local perturbation relative to the contact between the solid member and the substrate. So, the invention suggests that the spacing members preferably comprise a solid end adapted to be arranged against the solid member, said solid end having a space therearound. Below, the description only refers to a capacitively coupled RF plasma reactor in which the improvements of the invention notably reduce the electromagnetic non uniformity during the plasma process.
First of all, for most processing plasmas, the electromagnetic propagation brings really a limitation in RF plasma processing for substrate sizes of the order, or larger than 0.5 m.sup.2 and especially larger than 1 m.sup.2, while the frequency of the RF source is higher than 10 MHz. More specifically, what is to be considered is the largest dimension of the substrate exposed to the plasma. If the substrate has a substantially square surface, said "largest dimension" is the diagonal of the square. So, any "largest dimension" higher than substantially 0.7 m is critical.
A basic problem, which is solved according to the present invention, is that, due to the propagative aspect of the electromagnetic wave created in the plasma capacitor, the RF voltage across the process space is not uniform. If a RF source is centrally connected to an electrode, the RF voltage decreases slightly from the center to the edges of said electrode.
As above-mentioned, one way to recover a (substantially) uniform RF voltage across the plasma itself, is the following: